1) Field of the Invention
The present invention relates to a thermal transfer image-receiving sheet. More particularly, the present invention relates to a thermal transfer image-receiving sheet capable of receiving and fixing thereon thermally transferred dye or ink images or pictures in a clear and sharp form without a thermal curling thereof, to record thereon continuous tone full-colored images or pictures at a high resolution and a high tone reproductivity, and capable of being smoothly moved through a thermal printer without fear of jamming.
2) Description of the Related Arts
Currently there is enormous interest in the development of new types of color printers capable of recording clear full color images or pictures, for example, relatively compact thermal printing systems, especially sublimating dye thermal transfer printers.
The small sized thermal dye transfer full color printers are expected to be widely utilized as printers for electronic camera and video printers.
In the dye thermal transfer printer, colored images or pictures are formed by superimposing a dye ink sheet composed of a substrate sheet and a dye ink layer formed on the substrate sheet and comprising a mixture of a sublimating dye with a binder on a dye image-receiving sheet composed of a dye image-receiving resinous layer formed on a substrate sheet in such a manner that the ink layer surface of the ink sheet is brought into direct contact with the dye image-receiving resinous layer of the dye image-receiving sheet, and the dye ink layer is partly heated by thermal heat of a printer in accordance with an input of electric signals corresponding to the images or pictures to be printed, to thermally transfer the dye images or pictures to the dye image-receiving resinous layer.
It is known that a dye image-receiving sheet composed of a substrate sheet consisting of a biaxially oriented film comprising a mixture of a polyolefin resin with an inorganic pigment or a synthetic resin paper-like sheet, and a dye image-receiving layer comprising a dye-receiving polymeric material, for example, a polyester resin, polycarbonate resin or acrylic resin, is useful for recording thereon clear dye images, using the thermal printer as mentioned above. The above-mentioned film or sheet has a uniform thickness, a high flexibility and a low thermal conductivity, compared with that of a wood pulp paper sheet, and therefore, is advantageous in that thermally transferred colored images thereon have an uniform image quality and a high color density.
Nevertheless, when dye images are thermally transferred to a dye image-receiving sheet having a substrate sheet composed of a biaxially oriented polyolefin film, the color density and uniformity of the resultant dye images are sometimes uneven, depending on the type of the substrate sheet, and therefore, the commercial value of the dye-image receiving sheet is not always constant. Namely, some dye image-receiving sheets are unsatisfactory in that a formation of uneven images and an insufficient sensitivity thereof occur due to an influence of the pigment.
Generally, the biaxially oriented sheet composed of a multi-layered polyolefin resin film containing an inorganic pigment has a uniform quality and exhibits a satisfactory conformity to the thermal head of the printer. Nevertheless, this type of synthetic resin paper-like sheet contains a relatively large amount of the inorganic pigment, and has a paper-like surface layer formed by a drawing operation, and having a number of voids. The paper-like surface layer has a relatively high roughness, and therefore, it is difficult to attain a high resolving power on the order of 10 .mu.m or less when using the above-mentioned conventional type of the synthetic paper sheet.
It is possible to increase the resolution and the reproductibility of the images to a certain extent, by increasing the pressure between a platen roll and a thermal head, but when this pressure of the platen roll becomes too high, the accuracy of the transferred images is lowered.
Also, due to a relatively high rigidity of the polyolefin resin in the synthetic paper sheet, there is a limitation of the degree of close contact of the image receiving sheet with the printing thermal head. Therefore, an improvement of the substrate sheet to enhance the quality of the thermally transferred dye images is strongly demanded.
Accordingly, the present invention intends to provide a substrate sheet from which a dye image-receiving sheet free from the above-mentioned disadvantages can be obtained.
Also, it is known that, in the dye thermal transfer image printer, a large amount of heat energy is imparted to the dye image receiving sheet, which causes an undesirable thermal shrinkage, curling and wrinkling of the image receiving sheet.
Where an oriented polymeric film is laminated and bonded to a core sheet having a small thermal shrinkage, the resultant dye image-receiving sheet exhibits a reduced thermal curling property in a thermal printing process, but, this type of core sheet is not satisfactory when trying to obtain a dye image-receiving sheet having a smooth movability in the thermal printer and capable of displaying high quality colored image thereon.
In a conventional dye image-receiving sheet, a dye image receiving resinous layer is formed on a substrate sheet. This dye image-receiving resinous layer usually comprises a resinous material, for example, saturated polyester resin, capable of being dyed with sublimating dyes.
Japanese Unexamined Patent Publication No. 58-215,398 discloses that the saturated polyester resin in the dye image-receiving layer is cross-linked with a cross-linking compound, for example, isocyanate compounds, to prevent a thermal melt-adhesion of the dye image-receiving layer with a dye ink layer of a dye ink sheet when a thermal transfer of the dye images is carried out from the dye ink layer to the image receiving layer by heat from a thermal head. When the cross-linking agent is added, the resultant dye image-receiving saturated polyester resin layer exhibits an increased thermal shrinkage depending on the type and the amount of the added cross-linking agent, and therefore when heated, the resultant dye image-receiving sheet is thermally curled in such a manner that the dye image-receiving resinous layer becomes an inside layer thereof.
The curling of the image-receiving sheets causes the travel of the sheets in the printer to be obstructed, and sometimes makes a delivery of the sheets from the printer impossible. Also, the quality of the printed colored images becomes poor.